This invention relates to dielectric bond plies and methods of manufacture thereof useful in the formation of circuits and multi-layer circuits.
Circuit materials are used in the manufacture of circuits and subassemblies and multi-layer circuits, and include circuit laminates, bond plies, resin coated conductive layers, free films, and cover films. Circuit materials are formed from dielectric materials that can include thermosetting and thermoplastic polymers. The polymers may be combined with fillers such as silica to adjust the dielectric or other properties of the polymers. One or more of the polymers in a bond ply is designed to soften or flow during manufacture of the circuit laminate or multi-layer circuit but not in use of the circuit.
A circuit laminate is a type of circuit material that has a conductive layer fixedly bound to a dielectric substrate layer. Double clad laminates have two conductive layers, one on each side of the dielectric substrate. Patterning a conductive layer of a laminate, for example by etching, provides a circuit layer, and thus a circuit. Multi-layer circuits comprise a plurality of conductive layers, at least one of which contains a conductive wiring pattern. Typically, multi-layer circuits are formed by laminating one or more circuits together using bond plies and, in some cases, adhesive-coated conductive layers, in proper alignment using heat and/or pressure. In use, the bond plies, or portions thereof, flow and completely fill the space and provide adhesion between circuits and/or between a circuit and a conductive layer, or between two conductive layers. In such multi-layer structures, after lamination, known hole-forming and plating technologies may be used to produce useful electrical pathways between conductive layers.
The bond plies used in the formation of rigid circuit laminates, multilayer circuits, and subassemblies, are often comprised of a glass fabric saturated with an uncured or B-staged polymer composition, which cures in the circuit or subassembly lamination process. During lamination, the polymer in this type of bond ply flows under heat and pressure into the features, e.g. conductor lines and spaces, on the surfaces of the opposing circuit layers being laminated together. The glass fabric provides a hard stop, which prevents the conductors on opposing layers from coming too close to each other causing low resistance, other reliability problems, and in the extreme, shorting. The glass fabric also helps control the horizontal, i.e. X-Y, flow of the polymer composition during lamination. Excessive X-Y flow can result in incomplete fill between conductor lines, which in turn can lead to multiple problems in further circuit fabrication and use.
Polymers used in bond plies are most often epoxies, but for improved electrical or thermal performance can be other polymers, e.g. polybutadienes, polyimides, polyphenyleneethers, or polybenzoxazines. Bond plies can be made with fabrics other than those based on glass, but these are used infrequently because they are expensive, have fabrication and use issues, or both. There are significant problems in the use of such fabric-based bond plies for dense and high speed, high electrical performance applications. The optimum design of a bond ply for such applications would be a structure in which the composition is homogeneous and has the same low dielectric constant, low dissipation factor and good thermal and mechanical properties throughout the bond ply structure. Such a homogeneous structure is not possible with bond plies based on glass fabric, because the glass usually has a higher dielectric constant and dissipation factor and very different mechanical properties from most polymers. Moreover, dense electrical circuits and subassemblies require very thin bond plies, e.g. less than 3 mils, which are costly and problematic to make, if they can be made at all, with very thin glass fabrics. Further, when glass fabric is used where there is a high density of Z-axis interconnects between layers in multilayer circuits, reliability problems can be caused by the hole forming processes. One such problem, conductive anodic filament growth, can cause low resistance and even shorts between closely spaced interconnects. This growth often occurs along the interface between the glass fibers and the cured polymer.
For these and other reasons, glass fabric based bond plies are not appropriate for dense, high electrical performance multilayer and subassembly applications. An alternate bond ply, which contains no glass or fabric, is comprised of a three layer structure. This bond ply, used most often in multilayer flex circuits, is comprised of a core dielectric layer which is a polymer film with uncured or B-staged dielectric layers on both sides. In fabrication and use the core layer is a film which does not melt or flow. During lamination to make multilayered structures, the outer dielectric layers flow and fill circuit features on the surfaces opposite to those being laminated to adhere the layers together, while the core layer provides a hard stop. Such film-based bond plies are sometimes used in dense multilayer structures. These film-based bond plies often use a polyimide film, such as Kapton®, with the adhesive outer layers being uncured or B-staged epoxy or acrylic. Fillers are sometimes used in the adhesives to help control flow on lamination.
These film-based bond plies have some of the same deficiencies as glass fabric-based bond plies in dense or high performance multilayers and subassemblies where the best electrical performance is required, because the base film and the adhesive layers have significantly different electrical and mechanical properties. Again, the non-homogeneity of the structure leads to reduced electrical performance and to reliability problems. Sometimes, uncured or B-staged polymer dielectric, often epoxy or acrylic-based polymer, is used alone as a bond ply. Such an adhesive-only bond ply can be homogeneous, but lamination of such bond plies to make multilayer structures is problematic due to the absence of a hard stop. The absence of a hard stop makes it difficult to accurately control conductor layer to conductor layer distances on lamination, which can lead to shorting and other electrical problems. Also, without glass fabric or a non-melting film core to help control X, Y, and Z flow on lamination, problems of too little flow, leading to voids between circuit conductors, or of too much flow, leading to squeeze-out, can occur, resulting in circuit reliability issues.
There accordingly remains a need in the art for a thin, homogeneous bond ply which does not include a glass fabric, has excellent electrical properties, in particular, low dielectric constant and low dissipation factor, good thermal and mechanical properties, and offers a hard stop. For optimum performance in dense, high speed, high signal integrity applications, the homogeneity should extend in all dimensions: X, Y and Z. These needs are not currently met by glass fabric-based bond plies, by existing film-based bond plies, or by adhesive-only bond plies.